The metal working process known as extrusion involves pressing metal stock (ingot or billet) through a die opening having a predetermined configuration in order to form a shape having indefinite length and a substantially constant cross section. In the extrusion of aluminum alloys with which this invention is particularly concerned, the aluminum stock is preheated to the proper extrusion temperature. It is then placed into a heated cylinder. The cylinder utilized in the extrusion process has a suitable die at one end which has art opening of the desired shape and a reciprocal piston or ram having approximately the same cross-sectional dimensions as the bore of the cylinder. This piston or ram moves against the stock to compress the stock. The opening in the die is the path of least resistance for the billet under pressure, and metal deforms and flows through the die opening to produce a continuous extruded product having the same cross-sectional shape as the die opening.
The extrusion process generates a considerable amount of heat, and as a result, the temperature of the extruded product may vary during the extrusion process. Heat can be a desirable by-product in that the hotter the metal the more deformable the metal becomes. Initially, the die and surrounding parts of the extrusion press act as a heat sink. As the process proceeds, the temperature gap is reduced between the die, and the billet and the die and extrusion press stop acting as a heat sink. Then, the temperatures of the billet and the die both begin to rise. In addition, if the extrusion speed is high, heat may not be dissipated fast enough, and the temperature of the billet rises.
When extruding a series of billets, the temperature of the extrusion product can vary from different billets. In some instances, the rise in temperature may be sufficient to melt or at least weaken the metal to the point at which the frictional stresses at the surface cause cracking. Therefore, after the billet has reached a maximum temperature, the heat begins to become an undesirable by-product of the extrusion process from the standpoint of producing commercial quality extruded product.
The present invention is particularly concerned with hard aluminum base alloys. Extruded profiles of hard aluminum alloys have considerable commercial value. Such alloys find diversified use as structural materials and are used in the aeronautics industry because of their very high strength-to-weight properties. In order to produce extruded articles from such alloys in the most economical manner, the extrusion process should be carried out at the highest extrusion speed possible for the extrusion press being used.
Extrusion pressures, speed and temperature are factors that affect the quality of hard alloy,s as extruded products. Extrusion speed is usually referred to in terms of the progression of extruded material exiting from the die in linear feet per unit of time (minute or hour) or in terms of the progression of the ram (ram speed) pushing against the metal stock. In order to achieve acceptable surface quality in extruded hard aluminum alloy products, a certain limited range of extrusion speeds and temperatures must be closely observed, with the range being related to the size and complexity of the extrusion, the composition of the alloy and the reduction in cross-sectional area of the metal stock during the extrusion process.
Exceeding the predetermined speed and temperature ranges generally causes a rupture of the extrusion surface and also other defects such as recrystallization, blistering and broken surface, which result in rejection of the extruded product. High strength aluminum alloys must be extruded more slowly and at lower temperatures than lower strength aluminum alloys in order to avoid surface cracking of the high strength alloys with the resulting decrease in productivity. In addition to surface quality, the temperature of the billet must be kept above a certain minimum temperature to avoid phase changes in the crystal structure of the extrusion product which could greatly change the strength characteristics of the final extruded product.
Furthermore, a typical extrusion plant often has thousands of different dies that produce thousands of different shapes. The same alloy behaves differently for each of the thousands of dies and requires different preheat temperatures and extrusion rates depending on the die ratio, die layout and billet lengths. In the art, the extrusion dies are typically classified into groups depending on features, such as complexity, wall thickness and shape, to try to predict how to best extruded an alloy.
The extrusion conditions (speed and temperature) of hard aluminum alloys are determined empirically and kept below safe speed and temperature limits by experience to reduce the risk of impairing the quality of the extruded product. If these safe limits are set too low, the productivity and thus the profitability of an extrusion plant may be unnecessarily placed in jeopardy. In addition, when operating within the empirically determined safe limits, often there is considerable variation within an extruded piece and from piece to piece for the same shape.
Thus, it can be seen that it would be of great advantage, particularly in high strength aluminum alloys, if the properties of an extruded product and the productivity of an extrusion press can be improved together.
It was against this background that the development of the present invention came about.
The primary object of the present invention is to improve the productivity of an extrusion press.
Another object of the present invention is to improve the productivity of an extrusion press without significantly decreasing the commercial quality of the product that is being extruded. The commercial quality of the extruded product is evaluated in terms of tensile and field strengths and grain structure.
Yet another object of the present invention is to provide a method and system for extruding high strength aluminum alloys at the highest possible extrusion speeds without loss of extruded product due to physical defects.
Another object of the present invention is to remove the guesswork out of predicting safe extrusion speeds and thus close the gap between the ideal safe speed and the actual operating speed of the extrusion press.
Still another objective of the present invention is to provide a method and apparatus that is capable of increasing productivity in extruding high strength aluminum alloys for a wide variety of shapes and sizes.
These and other objects and advantages of the present invention will be more fully understood and appreciated with reference to the following description.